A ROTARY ANODE FOR AN X-RAY SOURCE

Abstract

The rotatable anode of a rotating anode X-ray source has demanding requirements placed upon it. For example, it may rotate at a frequency as high as 200 Hz. X-ray emission is stimulated by applying a large voltage to the cathode, causing electrons to collide with the focal track. The focal spot generated at the electron impact position may have a peak temperature between 2000 C. and 3000 C. The constant rotation of the rotating anode protects the focal track to some extent, however the average temperature of the focal track immediately following a CT acquisition protocol may still be around 1500 C. Therefore, demanding requirements are placed upon the design of the rotating anode. The present application proposes a multi-layer coating for the target region of a rotating X-ray anode which improves mechanical resilience and thermal resilience, whilst reducing the amount of expensive refractory metals required.

Claims

1. An rotatable anode for a rotating-anode X-ray source, comprising: a substrate; and a target region formed on the substrate; wherein the target region comprises a multi-layer coating comprising a first layer of a first material deposited on a surface of the substrate, and a second layer of a second material deposited on the surface of the first layer; wherein a thickness ratio between the first and second layers of the multi-layer coating in the target region is between approximately 0.5 to 2.0; and wherein the first material has a greater mechanical resilience compared to the second material, and the second material is more thermally conductive compared to the first material.

2. The rotatable anode according to claim 1, wherein the thickness ratio between the first layer and the second layer in the target region is between approximately 0.95 to 1.05.

3. The rotatable anode according to claim 1, wherein the total thickness of the first layer and the second layer is between approximately 5 um to 60 um.

4. The rotatable anode according to claim 1, wherein the first material is one of rhenium, tantalum, tantalum carbide, and tungsten carbide.

5. The rotatable anode according to claim 1, wherein the second material is one of tungsten, iridium, and a tungsten-rhenium alloy.

6. The rotatable anode according to claim 5, wherein the second material is pure tungsten, and the second layer has a thickness between approximately 5 to 60 um.

7. The rotatable anode according to claim 1, wherein the surface of the second material in the target region is smoothed by a thermal sintering process at a temperature of greater than approximately 1500 C.

8. The rotatable anode according to claim 7, wherein the surface of the second material in the target region has a surface roughness lower than approximately 5 um.

9. The rotatable anode according to claim 1, wherein the target region is provided as a first area of the rotatable anode, and a non-target region comprises a second area of the rotatable anode, the first layer of the first material additionally deposited on the surface of the second area of the substrate.

10. The rotatable anode according to claim 1, wherein the substrate is formed from a carbon composite or graphite.

11. A rotary anode X-ray tube, comprising: an evacuated envelope; a rotatable anode comprising: a substrate; and a target region formed on the substrate, wherein the target region comprises a multi-layer coating comprising a first layer of a first material deposited on a surface of the substrate, and a second layer of a second material deposited on the surface of the first layer, wherein a thickness ratio between the first and second layers of the multi-layer coating in the target region is between approximately 0.5 to 2.0, and wherein the first material has a greater mechanical resilience compared to the second material, and the second material is more thermally conductive compared to the first material; and a cathode contained within the evacuated envelope, oriented to accelerate electrons towards the rotatable anode to cause X-ray emission.

12. The rotary anode X-ray tube according to claim 11, wherein the rotary bearing is a hydrodynamic bearing, which comprises a liquid metal lubricant, or a sliding bearing.

13. A method of manufacturing a rotatable anode, comprising: providing a rotatable anode substrate; depositing a first layer of a first material onto a surface of the substrate; and c) depositing a second layer of a second material on the surface of the first layer; wherein a thickness ratio between the first and second layers in the target region is between approximately 0.5 to 2.0; and wherein the first material has a greater mechanical resilience compared to the second material, and wherein the second material is more thermally conductive compared to the first material.

14. The method of manufacturing a rotatable anode according to claim 14, further comprising: sintering the rotatable anode substrate with first and second layers by heating to a temperature between approximately 1500 to 3200 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Exemplary embodiments of the invention will be described with reference to the following drawings:

[0051] FIG. 1 shows a schematic view of a conventional rotating anode X-ray source.

[0052] FIG. 2 shows a conventional rotating anode.

[0053] FIG. 3a) and b) show embodiments of a rotatable anode in accordance with aspects discussed herein.

[0054] FIG. 4 illustrates a manufacturing process in accordance with a third aspect of the invention.

[0055] FIGS. 5a) and 5b) show photographs of anode targets before and after a sintering process.

DETAILED DESCRIPTION OF EMBODIMENTS

[0056] FIG. 1 illustrates a conventional rotating anode X-ray tube 10. It comprises an external container 12 with a tube envelope 14 inside. A gap between the external container 12 and the tube envelope 14 is typically filled with an insulating fluid, such as oil. The tube envelope contains a rotatable anode disk 18, and a cathode 20 aligned with an outer perimeter of the rotatable anode disk 18. In operation, the cathode 20 emits electrons at high velocity towards the outer perimeter of the rotatable anode disk 18, and X-ray emission 22 out of the vacuum envelope occurs principally as bremsstrahlung emission. Only a small proportion of the high velocity electrons are converted into X-ray radiation, leaving the energy in the rest of the electron beam to be dissipated from the focal spot on the outer perimeter of the rotatable anode disk 18 as, typically, 50 kW to 100 kW of heat energy. For this reason, the rotatable anode disk 18 is rotated at frequencies as high as 200 Hz, to ensure that the target area (focal track) is not damaged by excessive heating.

[0057] In modern rotating anode X-ray tubes, a bearing system 24 is provided between an anode support shaft inside the tube envelope 14, and an outer rotor 26. Typically, this is a liquid metal bearing system to enable heat conduction from the rotatable anode disk 18 out of the vacuum envelope. Also present is a motor subsystem, comprising a stator 28 attached to the external container 12 and a rotor body 30 typically comprising a copper cylinder. In operation, energisation of the stator 28 causes the rotatable anode disk 18 to move around an axis defined by the bearing system 24.

[0058] FIG. 2 illustrates a conventional X-ray rotating anode target 32. The illustrated target is a segmented all-metal anode bearing a focal track region 34 which may, for example, comprise a tungsten-rhenium alloy of 1 mm thickness as its top layer. However, the use of such a thick refractory metal alloy significantly increases the cost of such a rotary anode.

[0059] Furthermore, the use of rhenium as a rough heat radiating coating means that the granular structure of the rhenium coating is disadvantageous from a thermal perspective, as the lateral heat conductivity is diminished compared with the bulk material of the anode. Furthermore, the quality and amount of X-radiation, which is typically taken off the anode at a grazing angle, is worsened through intrinsic attenuation and beam filtration.

[0060] FIG. 3a) illustrates a schematic of a rotary anode according to a first aspect in a side cut-through view through the axis of rotation.

[0061] According to the first aspect, there is provided a rotatable anode 40 for a rotating-anode X-ray source, comprising:

[0062] a substrate 42; and

[0063] a target region 44 formed on the substrate 42.

[0064] The target region comprises a multi-layer coating 46a, 46b comprising a first layer 46a of a first material deposited on a surface of the substrate 42, and a second layer 46b of a second material deposited on the surface of the first layer.

[0065] A thickness ratio between the first and second layers of the multi-layer coating in the target region is in the range 0.5 to 2.0.

[0066] More particularly, thickness ratio between the first layer 46a and the second layer 46b is in the range 0.95 to 1.05, or in the range 0.6 to 1.5, or in the range 0.75 to 1.25.

[0067] Optionally, the total thickness of the first layer 40a and the second layer 40b is in the range 5 m to 60 m, in the range 20 m to 55 m, or in the range 30 m to 52.5 m.

[0068] The target region is provided with a multi-layer coating comprising two materials which may be selected to have complimentary properties in operation. For example, the first material is a material having relatively high mechanical stability at high temperature and stress compared to the second material such as rhenium, tantalum, tungsten carbide, or tungsten carbide. Rhenium additionally functions as a diffusion barrier between a carbon anode substrate and the tungsten layer, for example.

[0069] The second material may, for example, be a material having a higher thermal conductivity compared to the first material, for example tungsten or iridium. Optionally, the second material is pure tungsten, and the second layer has a thickness in the range of 5 m to 60 m, 10 m to 50 m, 15 m to 45 m, 20 m to 35 m, 22.5 m to 27.5 m. FIG. 3a) illustrates an example schematic of a rotary anode according to an optional embodiment of the first aspect in a side cut-through view through the axis of rotation.

[0070] The target region 44 is provided as a first area 48 of the rotatable anode, and a non-target region 50a, 50b comprises a second area of the rotatable anode, the first layer of the first material additionally deposited on the surface of the second area of the substrate 42. In other words, a microscopic layer 46a of a first material (for example, rhenium) extends substantially over the focal track of the rotatable anode 42, and a second microscopic layer 46b of tungsten is provided on top of the layer of the first material in the target region (focal track).

[0071] Optionally, substrate 42 is formed from carbon composite or graphite.

[0072] Optionally, the surface of the second material is smoothed by a thermal sintering process at a temperature of optionally greater than 1500 C., greater than 2000 C., or greater than 2250 C., or greater than 2500 C. or greater than 2750 C.

[0073] Accordingly, after thermal sintering, the surface roughness of the second material in the target region may be lower than 5 m, meaning that a further surface smoothing step (for example, performed by machining) is not required.

[0074] As a preferred embodiment, the first material is provided as a layer of pure rhenium having a thickness ranging between 20 m to 25 m, and the second material is provided as a layer of pure tungsten having a thickness ranging between 20 m to 25 m. Advantageously, the rhenium has superior mechanical performance to that of tungsten, and can perform as a diffusion barrier for carbon. The tungsten has a superior thermal performance compared to the rhenium, and functions to spread heat more quickly to areas of the focal track that are not in the direct instantaneous path of the electron beam. The relative thinness of both of the rhenium and tungsten layers (when compared with the typical case of a 1 mm thick rhenium layer, for example) means that tensile stresses caused by thermal expansion and contraction are minimized, compared to the use of thicker rhenium and/or tungsten layers. Furthermore, cracks appear less quickly, compared to conventional all-rhenium surfaces.

[0075] From a metallurgical perspective, the microscopic surface of rhenium comprises many irregularities which protrude tens of m from the substrate surface (seen, for example, in FIG. 5a. The use of tungsten as a second material layer enables the tungsten to spread around the protrusions of rhenium, improving the smoothness of the rotary anode. Optionally, the target region 44 is provided as a first area 48 of the rotatable anode, and a non-target region 50 a, 50b comprises a second area of the rotatable anode.

[0076] FIG. 3b) illustrates a schematic side-view of a rotary anode according to an optional embodiment of the first aspect in a cut-through view through the axis of rotation. In FIG. 3b), reference numerals are common to FIG. 3a), where appropriate.

[0077] Optionally, and as illustrated in FIG. 3b), the target region 44 is provided as a first area 48 of the rotatable anode, and a non-target region 50a, 50b comprises a second area of the rotatable anode, the first layer of the first material additionally deposited on the surface of the second area of the substrate 42. In other words, a microscopic layer 46a of a first material (for example, rhenium) extends substantially over the entire upper surface of the rotatable anode 42, and a second microscopic layer 46b of tungsten is provided in the target region (focal track). Advantageously, the roughened surface of the rhenium exposed in the non-target region is a better heat radiator than the bare anode substrate.

[0078] Optionally, the target region 44 extends into the non-target region by 5%, 10%, or 15% of the width of a focal spot to provide a safety margin, such that the microscopically thin rhenium layer is not damaged by direct exposure to the electron beam.

[0079] According to a second aspect, there is provided a rotary anode X-ray tube comprising:

[0080] an evacuated envelope;

[0081] a rotatable anode in accordance with the first aspect or its embodiments supported on a rotary bearing contained within the evacuated envelope; and

[0082] a cathode contained within the evacuated envelope, oriented, in operation, to accelerate electrons towards the rotatable anode to cause X-ray emission.

[0083] The manufacture of a rotatable anode will now be discussed.

[0084] FIG. 4 illustrates a process for manufacturing a rotatable anode according to the first aspect.

[0085] The method of manufacturing a rotatable anode, comprises:

a) providing 60 a rotatable anode substrate;
b) depositing 62 a first layer of a first material onto a surface of the substrate; and
c) depositing 64 a second layer of a second material on the surface of the first layer. The thickness ratio between the first and second layers in the target region is in the range 0.5 to 2.0.

[0086] Step a) of providing a rotatable anode substrate optionally comprises obtaining a circular carbon (carbon felt or composite) or graphite blank and placing it in a suitable chemical vapour deposition (CVD) reaction chamber.

[0087] Step b) comprises the deposition, for example by chemical vapour deposition, of a first layer of a first material on the substrate blank, to generate a substrate intermediate. Optionally, the first material is rhenium, optionally deposited to a thickness of 25 m. Following the deposition of the first material, the CVD reaction chamber is purged in preparation for subsequent step.

[0088] Although CVD has been referred to above, any suitable material deposition approach may be used in the manufacturing method. For example, pulsed laser deposition (PLD), plasma spraying (PS), physical vapour deposition, and electroplating are provided as nonlimiting examples of other manufacturing techniques applicable in steps a) and b).

[0089] Step c) comprises the deposition, for example by chemical vapour deposition, of a second layer of a second material on the substrate intermediate. Optionally, the second material is tungsten, optionally deposited to a thickness of 25 m.

[0090] Typically, there are intermediate steps of masking the substrate or substrate intermediate, to ensure that the first and second materials are deposited only on a target region (focal track). Optionally, the masking step is not applied before step b), such that a microscopic rhenium layer is provided across a substantially the whole upper surface of the anode blank.

[0091] Optionally, there is provided the step d) of sintering the rotatable anode substrate by heating it to a temperature in the range of 1500 C. to 3200 C., preferably to 1800 C. The effect of the sintering operation is to smooth the surface of the second material. Typically, sintering may be performed using an electron beam (optionally, the electron beam of the X-ray tube itself, before degassing and vacuum evacuation). Effectively, during manufacture, the focal track is smoothed by generating a focal spot having a temperature significantly higher than the focal spot applied during normal operation of the rotary anode.

[0092] Step d) is effectively a break-in process that can be combined with the tube anode heat testing step performed by a tube anode manufacturer. However, over driving the focal spot temperature during the break-in process enables the surface of the target region to have a low roughness.

[0093] Optionally, an unsintered area of the coating has a maximum roughness (Ra) of around 10 m and a sintered area of the coating has a maximum roughness of around Ra=4 m.

[0094] FIGS. 5a) and 5b) are images of, respectively, a pure rhenium CVD coating before (FIG. 5a) and after (FIG. 5b) the thermal sintering process of step d). As shown, before processing, the rhenium surface is relatively rough, whereas following the thermal sintering treatment, the rhenium surface is smoother.

[0095] It should to be noted that embodiments of the invention are described with reference to different subject-matters. In particular, some embodiments are described with reference to method-type claims, whereas other embodiments are described with reference to device-type claims. However, a person skilled in the art will gather from the above, and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject-matter, other combination between features relating to different subject-matters is considered to be disclosed with this application.

[0096] All features can be combined to provide a synergetic effect that is more than the simple summation of the features.

[0097] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood, and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

[0098] In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor, or other unit, may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.